1)
Why did he say "Chakreni Kel v'Da Levavi"?
1.
Radak: Because he mentioned that Resha'im have evil intent in Emunah in Hashem, he says, investigate my heart - You will see that it is straight in Your Emunah.
2.
Malbim: This refers to deeds that their source is in the heart, which desires and arouses actions.
2)
What are "Sar'apai"?
1.
Radak: My thoughts. This is like "Se'ipai" (Iyov 20:2). Both of these refer to thought, just like in a tree the branches are called "Sar'aposav" and "Se'aposav (Yechezkel 31:5, 8) for thoughts are to the heart like branches to a tree.
2.
Malbim: They are my thoughts in matters of Emunah and mindset.
